Prior to the present invention, various procedures were used to cure epoxy resins. For example, Crivello, U.S. Pat. No. 4,058,401, shows that epoxy resins can be cured by use of a photoinitiator, such as a triarylsulfonium salt, in combination with actinic radiation such as ultraviolet light. Heat curable epoxy resins are often preferred in particular applications, particularly where exposure to radiation to activate a photoinitiator is not feasible. The use of anhydride cured epoxy resins in the electrical industry is widespread for insulating electrical parts due to the excellent mechanical and electrical properties of the cured resin. In U.S. Pat. No. 4,511,701 (Ryang), there is shown the use of a silyl norbornane functional anhydride which has been found to provide excellent physical properties when used in combination with a small amount of organic amine such as benzylethylamine. However, a mixture of the epoxy resin, anhydride, and amine accelerator cures rapidly on mixing. In the absence of an amine accelerator, a mixture of the silyl norbornane anhydride and the epoxide resin has been found to be stable for a year under ambient conditions.
Latent cures of epoxy resins also have been used to some degree. For example, boron trifluoride-monoethylamine complex has been used as well as various metal salts and a variety of phosphonium and ammonium salts as shown by U.S. Pat. Nos. 3,784,583 and 3,978,026. A further strategy to use a latent amine is the employment of urethanes as shown by U.S. Pat. No. 4,111,917 but the expulsion of CO.sub.2 makes this system impractical for many applications. The use of metal acetylacetonates is shown by U.S. Pat. No. 4,254,351.
It would be desirable, therefore, to provide a heat curable mixture of epoxy resin and anhydride which would be stable under ambient conditions for an indefinite period of time, such as 6 months or more, and which would cure within 30 minutes at a temperature in the range of from 150.degree. C. to 200.degree. C.
The present invention is based on the discovery that certain amine adducts of cobalt (II) acetylacetonate and, preferably, secondary diamine adducts, for example, the adduct with di-N-butyl ethylenediamine has been found to provide shelf stabilities in excess of 6 months under ambient conditions and can be converted to a thermoset within 3 minutes at 190.degree. C.